1. Field of the Invention
This invention pertains to refining of lubricating oils. In particular, it pertains to reclaiming refined lubricant base stock oils which fail to meet specifications because of development of wax haze on storage.
2. Prior Art
High quality lube base stock oils are conventionally prepared by refining distillate fractions or the residuum prepared by vacuum distilling a suitable crude oil from which the lighter portion has been removed by distillation in an atmospheric tower. Thus, the charge to the vacuum tower is commonly referred to as a "long residuum", and the residuum from the vacuum tower is distinguished from the starting material by referring to it as the "short residuum".
The vacuum distillate fractions are upgraded by a sequence of unit operations, the first of which is solvent extraction with a solvent selective for aromatic hydrocarbons. This step serves to remove aromatic hydrocarbons of low viscosity index and provides a raffinate of improved viscosity index and quality. Various processes have been used in this extraction stage, and these employ solvents such as furfural, phenol, sulfur dioxide, and others. The short residuum, because it contains most of the asphaltenes of the crude oil, is conventionally treated to remove these asphalt-like constituents prior to solvent extraction to increase the viscosity index.
The raffinate from the solvent extraction step contains paraffins which adversely affect the pour point. Thus, the waxy raffinate, regardless of whether prepared from a distillate fraction or from the short residuum, must be dewaxed. Various dewaxing procedures have been used, and the art has gone in the direction of treatment with a solvent such as MEK/toluene mixtures to remove the wax and prepare a dewaxed raffinate. The dewaxed raffinate may then be finished by any of a number of sorption or catalytic processes to improve color and oxidation stability.
The quality of the lube base stock oil prepared by the sequence of operations outlined above depends on the particular crude chosen as well as the severity of treatment for each of the treatment steps. Additionally, the yield of high quality lube base stock oil also depends on these factors and, as a rule, the higher the quality sought, the less the yield because the refining steps are subtractive operations. Because of the yield penalties and the relatively high cost of the unit processes themselves, refined lube base stock oils are costly.
It sometimes happens in the course of routinely refining a lube oil that a product is made that is up to specifications except for some deficiency suggesting contamination by a small amount of high melting wax. For example, a refined oil may be prepared which is clear and bright and which has a satisfactory cloud point and pour point; however, on storage at a low temperature which is higher than the cloud point, a wax haze develops which makes the oil commercially unacceptable. In such a situation, it may be found, for example, that if the wax removed in the solvent-dewaxing operation is blended back with the oil and the dewaxing operation repeated, the product no longer develops wax-haze during storage.
The production of refined, dewaxed oils which do not meet specifications due to wax-contamination can occur is several ways. A small tear in the filter cloth on a drum filter, for example, may lead to a minor contamination by wax. Storage of the oil in vessels contaminated by wax is another possible source of the problem. One may speculate on other reasons for the occasional apparent wax-contamination in a process which normally produces acceptable lube oils, but such speculation is not particularly pertinent in the present context, which is concerned with reclaiming such wax-contaminated oils.
The refiner takes a severe economic penalty when a dewaxed oil is produced that fails some specification, such as a haze test, because the failure usually is discovered only after all the raw material and process costs have been expended to make the product. No way appears to be known that will effectively and economically remove the minute amount, usually less than about 2 wt.%, of wax contaminant. Contaminated oils that cannot in a practical way be mixed with acceptable oil to make an acceptable blend (i.e. "blended-off") may find no market or use other than to be fed to a catalytic cracking unit or burned as heavy fuel.
In recent years techniques have been proposed for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. No. Re. 28,398 is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938.
U.S. Pat. No. 3,755,138 discloses a two-step process for preparing low pour point lube oils, which process involves mild solvent dewaxing to an intermediate pour point followed by catalytic hydrodewaxing with a ZSM-5 type crystalline aluminosilicate zeolite.
The above-cited references all are concerned with removing substantial amounts of wax from waxy stocks with substantial reduction of the pour point of the oil.
It is an object of this invention to provide an inexpensive catalytic process for reclaiming refined lubricating oils contaminated by wax. It is a further object of this invention to provide a simple catalytic process for reclaiming, without substantial change of the pour point and other specifications of the oil, a dewaxed lube base stock oil having an excessive tendency to form particulate wax when stored. Other objects will be evident to those skilled in the art upon reading this entire specification including the claims thereof.